1. Field of the Invention
The present invention relates to a pixel driving device and a light emitting device.
2. Description of the Related Art
Research and development has been gaining in popularity in recent years around light emitting element type display devices (light emitting element type display, light emitting device) that provide a display panel (pixel array) arranging light emitting elements in a matrix as the next generation of display device to succeed liquid crystal display devices.
Electric current driven type light emitting elements, such as organic electroluminescence elements (organic EL element) and inorganic electroluminescence elements (inorganic EL element), or a light emitting diode (LED), are known as this type of light emitting element.
A light emitting element type display device that applies an active matrix drive method, compared to known liquid crystal display devices, especially has characteristics which include faster display response speed, no viewing angle dependency, high brightness and superior contrast, and the ability for high resolution display picture quality.
In addition, a light emitting element type display device has an extremely advantageous characteristic in that further thinning of thin film becomes possible since, unlike a LCD device, a light emitting element type display device does not require a backlight or a light guide plate. Therefore, application on future electronics devices of this type is anticipated.
An organic EL display device with an active matrix driving method that controls electric current through voltage signals is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2002-156923 as this type of light emitting element type display device.
The organic EL display device with an active matrix driving method equips each pixel with an organic EL element that is a light emitting element and with a pixel drive circuit having a current control thin film transistor to drive the organic EL element as well as a switching thin film transistor.
The current control thin film transistor controls the current value of the electric current that flows between the drain and the source of the current control thin film transistor by an impressed gate voltage after a voltage signal is impressed having a voltage value determined based on the image data of each pixel (hereinafter written as “voltage value based on the image data”) on the current control terminal of the current control thin film transistor. This current, supplied to the organic EL element, causes the organic EL element to emit light. The switching thin film transistor executes switching to supply the voltage signal based on image data to the gate of the current control thin film transistor.
The properties of a current control thin film transistor in a display device constituted in this manner undergo chronological changes with use. Particularly, it is known that when the current control thin film transistor consists of an amorphous TFT (Thin Film Transistor), the threshold voltage Vth, which is one of the properties of that TFT, exhibits comparatively large chronological change.
Even impressing the current control thin film transistor gate with a voltage signal of the same voltage value for the same gradation value of image data with a constitution that controls the gradation of the displayed image by the voltage value of the voltage signal based on image data, the current value of the electric current that flows between the drain and the source of the current control thin film transistor changes when the threshold voltage Vth changes, thereby changing the brightness of the light emitted from the organic EL element of the display pixel with respect to the same gradation value of the image data.
Other property of a current control thin film transistor, for instance, irregularity in the current amplification factor β between pixels also affects the displayed image. The current value of the electric current that flows between the drain and the source of the current control thin film transistor is proportional to the current amplification factor β. Therefore, even if the threshold voltage of the current control thin film transistor for every pixel is the same, irregularity will occur in the current value of the electric current that flows between the drain and the source of the current control thin film transistor when irregularity happens in the current amplification factor β value originating in, for example, the manufacturing process, thereby creating irregularity in the brightness of the light emitted from the organic EL elements.
Irregularity in the current amplification factor is due to irregularity in mobility. Irregularity in mobility is especially prominent in low temperature polysilicon TFT's while this type of irregularity in amorphous silicon TFT's are comparatively low. However, even so, the affects of irregularity in mobility, i.e. current amplification factor β, originating in the manufacturing process cannot be avoided.
In this manner, changes to the threshold voltage Vth and irregularity in the current amplification factor β originating in the manufacturing process affect the image data reproducibility of the displayed image, namely, picture equality.